net: hns3: Refactor mac mtu setting related functions

This patch refactors mac mtu setting related functions,
normalizes the use of mps and mtu.

Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Jian Shen <shenjian15@huawei.com>
Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>